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Lots of people love seafood. But most people know the oceans are overfished, and no one wants to be an eco-jerk. So which types of fish are OK to eat, and which should we avoid?

The answer: generally small fish that reproduce quickly and don’t eat meat are OK. That means catfish, tilapia and carp, among others.

“It has to be low on the food chain,” Sylvia Earle, explorer in residence at National Geographic, said at panel discussion Tuesday at Fortune’s Brainstorm Green conference.

Fish high on the food chain – large carnivore fish like tuna, swordfish and salmon – should be avoided.

“Think about it, we don’t raise snow leopards to eat, or any other carnivore,” said Earle.

Other sustainable fishing experts, including those from Greenpeace, have issued similar guidelines to Earle’s.

Earle said Tilapia can reach maturity in a year, meaning that the population can reproduce faster and is generally more resilient to overfishing. It takes a salmon 3 years to reach maturity, she said, and that the tuna you’re eating can be 6 to 20 years old.

She said eating even lower on the food chain is better, like sticking to plants or food made from microorganisms.

For those skeptical of the veggie-only diet, she pointed to “green caviar,” which is made from seaweed instead of fish eggs.

“It tastes hauntingly like caviar, but it’s cheap,” she said.

Others on the panel took exception to the small fish only rule, saying that is is possible to raise larger fish in an environmentally sustainable manner.

“We have to feed people what they are going to want to eat,” said Neil Sims, president of Kona Blue Water Farms, a Hawaiian operation that raises carnivorous yellowtail fish in offshore waters. “People need to eat oily fish, and catfish and tilapia don’t really cut it.”

Sims said that while his fish do eat meat, they don’t have to, and that the firm is looking to feed them with vegetable protein or protein derived from algae used in biofuels production.

Protecting the oceans

Everyone on the panel agreed more needs to be done to protect the seas, which Simms described as looking beautiful from the shore but resembling a WW I battlefield beneath the surface.

Establishing marine reserves – huge areas offshore that are closed to fishing and other development – is a key step.

“It’s politically feasible, and smart,” said Kim Lopdrup, president of the restaurant chain Red Lobster. “But there isn’t a single government agency that regulates this stuff, and that needs to get fixed.”

Eliminating waste in the fishing industry is another step.

“Five pounds of marine life goes over the back of the boat, dead, for every one pound of shrimp,” said Simms.

“It’s like using a bulldozer to catch a squirrel,” said Earle.

But even if the oceans are no longer overfished, several on the panel and in the audience brought up the problem of toxic chemicals leeching into the ocean from man’s activities on land. Notably, mercury from burning fossil fuels, chemical fire retardants, and other toxic materials.

Earle said many first-born baby dolphins are born dead, because there are so many chemicals in the mother’s body. Beluga whales, she said, would have to be treated as hazardous waste if they were disposed of on land.

‘I’m sure fish can be raised in a sustainable way,” said Frances Beinecke, head of the Natural Resources Defence Council, who was sitting in the audience. “But the whole biochemistry of the ocean is changing.”